Your best bet for Sandpaper is:
UnitedAuto B&M
or
http://www.nortonautomotive.com/
most I would go is 1200 grit. no need for ooohhhh shinys, unless you've a troll!
Just as sneekypeet stated. However, before you begin it's proper run a razorblade along the top to see if it actually needs to be lapped (nickel-copper alloy conducts, a pure copper base isn't necessary). If it does then mark an X over the nickel plating with a permanent marker. You mark the...
Great article.. I would have written one myself, but never had the time to!
Also take a gander at this article! I had never laughed so much in my life...
http://hubpages.com/hub/How-to-destroy-your-CPU-by-lapping-it